Since organic pigments have abundant hue, and good properties for coloring performance and transparency, are extensively applied as a coloring agent in various fields such as printing ink, ink for ink-jet, color toner for electronic photograph, color filter, reflection type display, cosmetics, and plastics. Further, several kinds of organic pigments are being investigated as functional materials in technical fields, being different from the use as the coloring agent, e.g., photoelectronic property or EL property, and are used as a constitutive material by utilizing said properties. In the case to use pigment in above mentioned various applications, particle size of pigment, shape of pigment and crystal form of pigment are the important properties, which affects to dispersability, coloring ability, weather resistance, light resistance and heat resistance at processing (Document 1: Dyes and pigments 21(1993) 273–297). For example, in a liquid type product containing an organic pigment for printing ink or ink for ink-jet, it is very important factor to control particle size and crystal form of pigment and to arrange the particle size distribution, for the improvement of thixotopic ability, coloring ability and disperse stability in the preservation of pigment. Further, the control of particle size of pigment, crystal form, and uniformity of particle size of pigment are important factors which affect to electrostatic stability in the use of toner for electronic photograph pigment, coloring ability and dispersing stability in cosmetics, and light resistance and weather resistance. Furthermore, in the development of above mentioned functional materials, it is important to find particle size, crystal and uniformity of particles size, and characteristics as the functional materials such as photoelectronic property, EL property, and to optimize the related photoelectric conductive property as well as the development of pigment derivatives with suitable function.
It is ideal that a product having properties requested for the organic pigment can be obtained by improving a synthesis method of organic pigment, however, at the present time, the method for preparation of organic pigment is not achieved to the technical level to satisfy said ideal situation. In particular, since quinacridone pigments and phthalocyanine pigments are superior in stability and safety as a coloring agent, these pigments are expected to be used in the field of photoelectronic property and EL property, and a new method for preparation of organic pigment particles is investigated. However, in an organic pigment obtained by new method, it is necessary to purify the synthesized crude pigment to control the particle size so as to adjust the desired condition, and to adjust the crystal form to cause the required characteristics as the coloring agent and the functional materials through the solvent post-treatment process.
Since the purpose of these treatment processes for the crude pigment is coincident to the requirement as the pigment, these treatments are called as pigment formation treatments.
In the meanwhile, since these pigments are generally insoluble in normal solvent, the pigment formation treatment contains various processes such as grinding, crystallization from sulfuric acid solution, and washing of crystallized particles, therefore, are not sufficient from the view point of energy consumption and environmental suitability (or versatility). And from the view point of characteristics of product, these pigments are not so the ideal, that is to say, the particle size is enough small monodispersed.
The inventors of the present invention have continued a development and an investigation of the technique to prepare nanometer size particles of organic compound. And in said development, the inventors of the present invention have established a technique called as re-precipitation method (Document 2: JP6-79168 publication). Namely, solution of organic compound is prepared using relatively easy-handling organic solvent, said prepared solution is poured into solvent which is poor solvent for said organic compound and is compatible with the organic solvent used for the preparation of the solution, by vigorous stirring, and then nanometer size particles of the organic compound by reprecipitation are fabricated.
In [0008] of the document 2, there is a description that [On the other hand, acid pasting method characterized by dissolving organic compound which is insoluble in normal solvent such as phthalocyanine pigment in sulfuric acid, accompanied with reaction to some extent, then dispersing into water and obtaining fine particles by mixing is disclosed, for example, in “The Phthlocyanines II” p. 35–37, F. H. Moser et al (1983, issued by CRC Press). This is a specific method to obtain fine particles of pigment, and since a strong acid is used, kinds of applied material to be used are limited, and the purity is in general lowered.].
Further, the inventors of the present invention also proposed the method for preparation of sub micron fine crystals of quinacridone pigment from crude quinacridone pigment particles, using reprecipitation method from supercritical or sub-critical solution [Document 3, WO 02/092700 A1, 21.11.02 International Publication, JP Laid Open Publication 2002-580681, laid open on May 15, 2002 (JPA 2001-144706)].
In this document, there is a description that quinacridone pigments with nanometer size can be obtained, however, it is not complete as the industrial production method. As a competitor to supply said pigments, suppliers of India and China are starting to join in the market, therefore, the development of a technique aiming mass production of high quality product is becoming the necessitated condition to overcome the competition with the competitors.
As a technique which can replace with the technique using sulfuric acid, there is a Paper from Allide Chemical Co., Ltd., reporting that crude quinacridone is heated in 8 times volume of polyphosphoric acid (PAA) at 85° C. to 100° C. for 4 to 18 hours, poured into methanol of 20 times volume, boiled for 1 hour, after washing by water, bright red or purple pigment is obtained. Further, the Paper describes that when crude β type quinacridone is dissolved in PAA and regenerated by lower aliphatic alcohol, β type changes to γ type, however, if treated by modified alcohol, quinacridone can maintain β type. Furthermore, there is a Paper reporting that the important factors to affect the product are temperature to cause regeneration, purity of alcohol and adding speed of alcohol. [Document 4, Yosio Nagai, Hisao Nishi “Dye and Drug” Vol 13, p 81–107 (88–89), (1968)].
Still further, in 9–15 page of “Journal of the Imaging Society of Japan” Vol. 37, No. 4, there is a description the preparation of α type oxotitaniumphthalocyanine pigment (hereinafter, abbreviated term: Pc pigment) comprising, forming a charge generating layer in a photo-semiconductor by dispersing α type oxotitaniumphthalocyanine pigment into a binder resin, the particle size of the Pc pigment used is 0.1 μm or less, and by refluxing in acetonitrile for 20 hours or in chloroform for 10 hours to obtain Pc pigment with said particle size and desired crystal form (page 10).
However, above mentioned pigment formation treatments, which do not use sulfuric acid, are not sufficient from the view point of simplicity and productivity. Therefore, it is very important to develop a pigment formation treatment technique, which dissolve problems referring to environment and energy, and to obtain organic pigment having desired pigment particle size and crystal form.
The subject of the present invention is to provide a pigment formation treatment technique, which dissolves the problems of conventional technique, and is available from the view point of industrial production. The inventors of the present invention continued an eager investigation to dissolve said problems using various solvents, and found that the amide solvents heated at high temperature can dissolve organic pigments, which does not indicate solubility in the organic solvents, especially quinacridone pigment or phthalocyanine pigment at high concentration. And, found that high concentrated pigment dispersion can be prepared by pouring the obtained solution into poor solvent to the pigment, especially into sufficiently cooled poor solvent, by vigorous stirring. Thus, said subject can be dissolved.
Further, the inventors of the present invention have found accidentally that among amide solvents, 1-methyl-2-pyrrolidinone has a specific solvent property that indicates into layers of polycyclic aromatic compound such as coal, and dissolve it, and can dissolve organic pigment at high concentration by breaking strong intermolecular interaction caused between planar electron conjugated molecules. Said accidental discovery and re-precipitation method which investigated eagerly by the inventors of the present invention are combined, and said subject can be dissolved.